1. Field of the Invention
The present invention relates to a combined antenna mounted on a movable body such as an automobile and capable of receiving satellite waves and ground waves.
2. Description of the Related Art
Circularly polarized waves are widely used in systems for receiving satellite broadcasts on a movable body such as an automobile, and in recent years, in order to improve reception in a blind zone such as in the shadow of a building, the use of a satellite broadcast system has been considered to retransmit from the stationary satellite the same contents as the direct broadcast waves. As for the antenna suitable for such a satellite broadcast system, a combined antenna has been suggested having a combined structure including a patch antenna for receiving satellite waves and a helical antenna (or rod antenna) for receiving ground waves on the same printed board in the related art (See Japanese Unexamined Patent Application Publication No. 10-107542, third page and FIG. 1 thereof). This combined antenna may receive circularly polarized satellite waves by means of the patch antenna facing the ceiling and receive vertically polarized ground waves without disturbing the satellite waves propagating to the patch antenna by means of the helical antenna (or rod antenna) installed with its axial direction inclined to the vertical.
In the above-mentioned conventional combined antenna, the helical antenna (or rod antenna) for receiving ground waves should be formed to have a long length, which causes it to be unsuitable for a small and thin antenna necessary for a movable body such as an automobile. Furthermore, the circularly polarized antenna for satellite waves and the vertically polarized antenna for ground waves of the combined antenna are installed very close together on a printed board to implement the compact size thereof, so that directivity of one antenna is apt to be changed in the region near the other antenna due to the electromagnetic coupling between the circularly polarized antenna and the vertically polarized antenna, which also causes receiving sensitivity to be degraded in a specific direction.